a) Field of the Invention
The present invention relates to an image fiber which has a tiny outside diameter and is capable of forming images of high quality.
b) Description of the Prior Art
Image fibers of this type which are fabricated mainly with quartz are conventionally utilized in medical fiberscopes, such as blood vessel fiberscopes and pyeloscopes, as well as fiberscopes for industrial inspections.
An endoscope 1 which is illustrated in FIG. 1, for example, is known as a conventional endoscope using an image fiber. An image T' of an object T illuminated with an illumination optical system (not shown) is formed by an objective lens system 2, as illustrated in FIG. 1, on an end surface of incidence 3a of an image fiber 3 which is composed of a bundle of a plurality of fibers (hereinafter referred to as optical fibers.) The image T' formed on the end surface of incidence 3a is allowed to emerge from an end surface of emergence 3b of the image fiber 3 for observation through an eyepiece 4.
Along with the recent progress made for reducing diameters of the fiberscopes, research and development are being made to configure an endoscope with an outside diameter smaller than 1 mm so that the endoscope 1 is usable for observing the interiors of blood vessels.
The image fiber which is used in the endoscope described above cannot always form images of satisfactory quality since it has a very small outside diameter on the order of several hundred micrometers, comprises cores each having a diameter of several micrometers and uses two to three thousand pixels.
The image fiber having such a small outside diameter mainly employs a composition referred to as "fiber conduit" in which optical fibers are made integral and arranged in a common cladding.
FIG. 2 is a sectional view schematically showing a fiber conduit 5 which is composed of a plurality of cores 7 arranged apart from one another in a common cladding 6.
However, the conventional image fiber 3 which is composed as described above has the disadvantage that it has a large diameter as a whole and can hardly be flexed when the number of the cores 7 arranged in the cladding 6 is increased to enhance the resolution of the image fiber 3 itself. When the cores 7 are arranged with spacings narrower than several micrometers, on the other hand, light incident on one of the cores 7 shifts into adjacent cores 7. This fact causes the problem that the image fiber is apt to produce a phenomenon referred to as blurring of light or cross talk of light and allows transmitted images to be blurred.
This phenomenon becomes noticeable in particular in the case where the cores are arranged with spacings narrower than 10 micrometers to reduce the outside diameter of the image fiber 3. Therefore, it is necessary to use a cladding 6 which is thick enough between the cores 7 for preventing the phenomenon of cross talk among the cores.
In recent years, however it has been desired that even the image fiber having such a small outside diameter be equipped with a high density of pixels. It is necessary for satisfying this desire to reduce the diameters of the cores 7 and to thin the cladding 6 lying among the cores 7 so as to enhance a density of pixels (or a density of the cores 7).
When the cores 7 are arranged with narrower spacings and the cladding 6 is several times thinner than wavelengths of light, however, cross talk is caused by mode coupling among the fibers, thereby resulting in remarkable degradation in image quality.
For preventing such degradation in image quality from being caused by the cross talk, it is necessary to thicken the cladding 6, but such thickening of the cladding 6 will lower a ratio of an area occupied by the cores within a unit sectional area of the image fiber. This causes the defect that it becomes impossible to obtain bright images and a high density of pixels.
The necessity to prevent cross talk makes it difficult for the prior art to provide an image fiber having a high resolving power.